1. Field of the Invention
The present invention relates to copperplate machines for printing paper currency notably banknotes, in the form of sheets or reels.
This machine comprises in general a cylinder having secured thereto at least one engraved plate provided with cuts corresponding to the elements of the main drawing and other cuts of lesser depth and greater fineness corresponding to the elements of a so-called safety ground or foundation, an impression cylinder, at least one ink roller inked by an inking unit and cooperating directly with the plate for inking the cuts representing the elements of the main drawing, a device for wiping the plate and preferably another, so-called pre-wiping device together with another inking system for inking the cuts corresponding to the safety ground with several (at least two) colors.
2. The Prior Art
In order to improve the safety factor against the forging of paper currency of all kinds, manufacturers have been led to produce paper currency by using the copperplate technique exclusively, for it is much more difficult to forge paper currency printed only by copperplate method, therefore also the safety ground thereof.
For this reason the Applicants already proposed in their French Patent No. 1,595,894 a polychrome copperplate printing machine comprising one or a plurality of inking devices for inking deep cuts corresponding to the main drawing, and two or more inking devices for inking the fine or shallower cuts corresponding to the safety ground.
However, in this specific case the ground cut inking device comprises a stereotype revolving while in surface engagement with the printing plate and adjustably secured to a carrier roll to permit the accurate setting of a perfect register and the correction of said register. However, this machine which, theoretically, should permit of printing completely paper currency according to the copperplate method in a single pass, was extremely complicated due to the use of stereotypes which implies a heavy loss of time for properly registering them, an operation which can only be carried out manually, this further constituting a source of defects or inaccuracies.
On the other hand, since with the copperplate method the printing plate is etched to different depths (up to 200 or 250 microns), the selector inking roller must exert a relatively strong pressure against the surface of the printing plate so that all the cuts of variable depths be properly filled with ink and thus yield subsequently the desired printing tones of a same color, this constituting a unique feature characterizing the copperplate method.
Consequently, the ink roller to be pressed against the plate must be lined with a relatively soft and resilient material, such as rubber, plastic or the like, of about 60.degree. Shore, for transferring the ink from the roller to the copperplate. Besides, this ink roller must be capable of transferring ink of a predetermined color only to well-defined areas of the surface or the etched plate. Therefore, in this case, a roller was used the surface of which was cut out according to the contours of the areas to be printed in the color concerned, and this surface comprised relief areas for receiving the ink. However, considering the relatively soft material lining this ink roller and the relatively high pressure to be exerted thereby against the copper plate for properly filling the deepest cuts, the fineness of the areas corresponding to the relief areas is rather limited. Though this restriction does not constitute a serious inconvenience as far as the main drawing is concerned, inasmuch as in general the color areas representing this main drawing are relatively extended, on the other hand it has prevented so far the use of these resilient ink rollers for prinring safety grounds because the roller areas in which very fine cuts are made are obviously liable to undergo a distortion as a consequence of the pressure exerted thereon, so that the final quality of the print is impaired. A safety ground or foundation worthy of this name must consist of very fine lines and even points, and it is for this reason that up to now it was impossible ot obtain such a safety ground by using a very finely cut ink roller.
To avoid this inconvenience, the Applicants already proposed in their French Patent No. 1,259,111 a polychrome printing machine comprising inter alia a plate carrier cylinder cooperating with a plurality of inking units equal in number to the colors to be printed, each inking unit comprising a plainsurfaced inking roller of resilient material cooperating with a selector cylinder provided with relief areas corresponding to those to be printed in the predetermined color.
However, the system disclosed in the above-mentioned French Patent did not permit of inking deep cuts formed in the plate portions constituting the main drawing of a banknote or other paper currency. It appeared that it was impossible to print in a single pass a paper currency having a deeply etched main drawing and fine etched polychrome safety grounds. Moreover, assembling the various inking rollers was a complicated operation, and obtaining a perfect register was an extremely difficult task.